I. Field of the Invention
The present invention relates generally to cellular communication systems, and specifically to cellular communication systems for areas where radio-frequency signals have difficulty entering.
II. Description of the Related Art
In cellular communications systems there are typically regions where the coverage is difficult or incomplete, for example, within metal-framed structures and underground. One of the reasons for the difficult coverage is Faraday-cage type shielding, wherein radio frequency (RF) signals have difficulty penetrating an effectively closed conducting structure. Another reason is Rayleigh fading, which is generated by a signal traversing multiple paths between a transmitter and a receiver. Typically the multiple paths are caused by reflections and/or refractions of the signal by objects between the transmitter and the receiver. The multiple paths followed by the signal generate interference effects at the receiver, which effects manifest themselves as differences in measured signal strength at the receiver, the measured signal strength being a function of the different paths followed by the signal. Methods for improving the coverage in regions where Faraday-cage type shielding and Rayleigh fading occur are known in the art.
U.S. Pat. No. 5,404,570, to Charas et al, whose disclosure is incorporated herein by reference, describes a repeater system used between a base transceiver station (BTS) which is able to receive signals and a closed environment such as a tunnel, wherein the environment is closed off to transmissions from the BTS. The system down-converts a high radio-frequency (RF) signal from the BTS to an intermediate-frequency (IF) signal, which is then radiated by a cable and an antenna in the closed environment to a receiver therein. The receiver up-converts the IF signal to the original RF signal. Systems described in the disclosure include a vehicle moving in a tunnel, so that passengers in the vehicle who would otherwise be cut off from the BTS are able to receive signals.
U.S. Pat. No. 5,603,080 to Kallandar et al., whose disclosure is incorporated herein by reference, describes a plurality of repeater systems used between a plurality of BTSs and a closed environment, wherein the environment is closed off to transmissions from the BTSs. Each system down-converts an RF signal from its respective BTS to an IF signal, which is then transferred by a cable in the closed environment to one or more respective receivers therein. Each receiver up-converts the IF signal to the original RF signal. Systems described in the disclosure include a vehicle moving between overlapping regions in a tunnel, each region covered by one of the BTSs via its repeater system. Thus passengers in the vehicle who would otherwise be cut off from one or more of the BTSs are able to receive signals from at least one of the BTSs throughout the tunnel.
U.S. Pat. No. 5,765,099, to Georges et al., whose disclosure is incorporated herein by reference, describes a system and method for transferring an RF signal between two or more regions using a low bandwidth medium such as twisted pair cabling. In a first region the RF signal is mixed with a first local oscillator to produce a down-converted IF signal. The IF signal is transferred to a second region via the low bandwidth medium, wherein the signal is up-converted to the original RF signal using a second local oscillator. The local oscillators are each locked by a phase locked loop (PLL) in each region to generate the same frequency, the locking being performed in each loop by comparing the local oscillator frequency with a single low frequency stable reference signal generated in one region. The reference signal is transferred between the regions via the low bandwidth medium.
One of the methods for overcoming Rayleigh fading, is to use a plurality of spatially-diverse receiving antennas, relying on the fact that statistically the chance of destructive interference occurring at all the antennas for a given signal is small. Using a plurality of antennas (in many cases two antennas are sufficient) enables a corresponding diversity of received signals to be analyzed, and typically the strongest signal is chosen. In the case of two antennas, the received signals are referred to as main and diversity signals.
U.S. Pat. No. 5,513,176, to Dean et al., whose disclosure is incorporated herein by reference, describes a distributed antenna array within a region where reception is difficult. The performance of the antenna array is enhanced by utilizing the signal diversity which is generated by the antennas being spatially distributed. Each antenna has a differential time delay applied to signals received by the antennas, so that the diverse signals are also separated in time. The differentially-delayed signals are preferably down-converted to an intermediate frequency and are combined, and the combined signal is then transferred out of the region via a cable.
It is an object of some aspects of the present invention to provide methods and apparatus for improved coverage in regions where cellular communication is inherently difficult.
In preferred embodiments of the present invention, repeater apparatus for use in a cellular communications system comprises a master transceiver unit, which communicates, preferably over the air or alternatively via a cable, with a base transceiver station using master radio-frequency (RF) signals. The master transceiver unit is coupled by one or more cables to one or more slave transceiver units, which are situated in an environment substantially closed off electromagnetically from the environment wherein the base transceiver station is situated. The slave units comprise respective slave antennas, by means of which the slave units communicate with mobile cellular transceivers using slave RF signals. Most preferably, the slave units are located in a region where the master RF signals are not able to penetrate, such as within a building. Signals between the master unit and the slave units are transferred at intermediate frequencies (IF) via the cables.
In both master and slave units, the IF signals are generated by down-converting the respective RF signals, and the RF signals are recovered by up-converting the IF signals. The up-conversion and down-conversion are performed using the same local oscillator (LO) frequency in the master and the slave units. In order to generate the same LO frequency in all units, one local oscillator in the master unit generates the LO frequency, and the generated LO frequency is divided by an integer in the master unit. The divided LO signal is transferred by the cable to the slave units, wherein the divided LO signal is multiplied by the same integer to recover the LO frequency.
Transferring an integer-divided LO frequency throughout the system, for use as a reference, enables recovery of the undivided LO frequency by multiplying by the same integer. Furthermore, the integer-divided LO signal can be distributed with low loss over conventional, inexpensive cable. Thus, one local oscillator supplies the entire system with the same LO frequency, so that problems which might be caused by LO frequency differences within the system are obviated.
In some preferred embodiments of the present invention, the RF signals comprise code division multiple access (CDMA) signals, which include one or more pilot signals associated with cellular channels over which the mobile transceivers are to communicate. The slave units comprise respective transmission delay elements which add a differential time delay to signals transmitted from the master unit to the respective slave units. The differential time delay effectively adds diversity to the signals received by the slave units, so that reception of pilot tone signals transmitted by the master unit is improved. In some preferred embodiments of the present invention, the slave units comprise respective receive delay elements, which add a differential time delay to signals received by the master unit from the respective slave units, so that reception of signals from the slave units is improved.
In some preferred embodiments of the present invention, the local oscillator generating the particular LO frequency is not necessarily a high-stability oscillator, since any variations in the LO frequency are transferred throughout the system.
In some preferred embodiments of the present invention, a plurality of master units communicate with a respective plurality of groupings of one or more slave units, each master and slave grouping operating substantially as described hereinabove. Preferably, the plurality of master units (and their respective slave units) utilize the same LO frequency to generate the IF frequencies used for master-slave communication. Alternatively, each master unit and its respective slave units utilize different LO frequencies. By dividing the slave units among the master units, an overall noise figure of the system is reduced. Most preferably, at least some of the slave units provide signals which are used as diverse receive signals, so that overall reception by the slave units is improved.
In some preferred embodiments of the present invention, the system operates as a channelized repeater system transferring channelized signals between the master and slave units. Most preferably, one or more filters are placed within the one or more slave units or one or more master units for the purpose of filtering out unwanted signals, for example signals outside specific channels which are designated to be transferred between the master and slave units.
There is therefore provided, in accordance with a preferred embodiment of the present invention, repeater apparatus for conveying a radio-frequency (RF) signal into an environment closed-off to the RF signal, including:
a master transceiver unit, including:
a master port which receives the RF signal;
a local oscillator (LO), which generates a LO signal at a LO frequency;
a frequency divider which divides the LO frequency of the LO signal by an integer to produce a divided LO signal; and
a master mixer coupled to the master port and the divider which generates an intermediate-frequency (IF) signal responsive to the RF signal and the LO signal;
one or more slave transceiver units, each unit positioned within the environment closed-off to the RF signal and including:
a frequency recovery circuit, preferably a frequency multiplier, which generates a recovered LO signal at the LO frequency by multiplying the frequency of the divided LO signal by the integer;
a slave mixer coupled to the multiplier which generates a recovered RF signal responsive to the recovered LO signal and the IF signal; and
a slave port coupled to the slave mixer which receives the recovered RF signal therefrom and transmits the recovered RF signal into the closed-off environment; and
one or more cables coupled between the master transceiver unit and the one or more slave transceiver units which convey the IF signal and the divided LO signal between the master transceiver unit and the one or more slave transceiver units.
Preferably, the master port is a two-way port, and each slave port is a two-way port through which the respective slave transceiver unit receives an RF slave signal from the closed-off environment and mixes the RF slave signal with the recovered LO signal to produce a slave IF signal which is conveyed by the one or more cables to the master transceiver unit, wherein a recovered slave RF signal is generated and is conveyed to the master port for transmission therefrom.
Preferably, each slave transceiver unit includes a respective IF signal reverse path wherein the reverse path comprises a delay element which delays the IF signal in the reverse path by a predetermined time.
Preferably, the master transceiver unit includes an antenna which receives the RF signal from a base transceiver station and transfers the received RF signal to the master port.
Preferably, the master port receives the RF signal via a cable coupled to a base transceiver station.
Further preferably, each slave unit includes one or more antennas coupled to the respective slave port which transmit the recovered RF signal into the closed-off environment.
Preferably, the master unit transceiver includes a DC power supply which generates a DC level that is conveyed over the one or more cables to power the one or more slave transceiver units.
Preferably, the master unit transceiver includes a controller which controls the operation of the master transceiver unit and the one or more slave transceiver units.
Further preferably, the repeater apparatus includes a remote control unit which transfers control signals between the controller and an operator of the apparatus.
Further preferably, the controller generates modulated control signals which are conveyed by the one or more cables between the master transceiver unit and the one or more slave transceiver units.
Preferably, the apparatus operates in a cellular communications network at frequencies substantially in the range 800 MHz to 1900 MHz.
Preferably, the IF signal corresponds to one or more predetermined channels of a multiple-access communications network.
Preferably, the one or more slave transceiver units include a plurality of slave transceiver units coupled to the master transceiver unit in one or more daisy-chain topologies.
Further preferably, the one or more slave transceiver units include a plurality of slave transceiver units coupled to the master transceiver unit in one or more star topologies.
Preferably, the one or more slave transceiver units include a plurality of slave transceiver units coupled to the master transceiver unit in one or more hybrid star-daisy-chain topologies.
Preferably, each slave transceiver unit includes a respective IF signal forward path, wherein the forward path includes a delay element which delays the IF signal in the forward path by a predetermined time.
There is further provided, in accordance with a preferred embodiment of the present invention, a method for conveying a radio-frequency (RF) signal into an environment closed-off to the RF signal, including:
receiving the RF signal at a master port;
providing a local oscillator (LO) signal operating at a LO frequency in a vicinity of the master port;
dividing the LO frequency of the LO signal by an integer to produce a divided LO signal;
generating an intermediate frequency (IF) signal responsive to the RF signal and the LO signal;
conveying the IF signal and the divided LO signal to the closed-off environment;
multiplying the frequency of the divided LO signal by the integer to generate a recovered LO signal having the LO frequency;
mixing the recovered LO signal and the IF signal to generate a recovered RF signal;
coupling the recovered RF signal to a slave port; and
transmitting the recovered RF signal into the closed-off environment from the slave port.
Preferably, the method includes:
receiving a slave RF signal from the closed-off environment at the slave port;
mixing the slave RF signal and the LO signal to produce a slave IF signal;
recovering the slave RF signal by mixing the slave IF signal with the LO signal; and
transmitting the slave RF signal from the master port.
Preferably, receiving the RF signal includes receiving a cellular communications transmission at a frequency in the range 800 MHz to 1900 MHz.
Preferably, generating the IF signal includes producing an IF signal having a frequency substantially less than the frequency of the RF signal.
Further preferably, generating the IF signal includes producing an IF signal having a frequency substantially less than the LO frequency.
Further preferably, generating the IF signal includes producing an IF signal to correspond to one or more predetermined channels of a multiple-access communications network.
Preferably, conveying the IF signal to the closed-off environment includes delaying the IF signal by a predetermined time.
Preferably, mixing the slave RF signal and the LO signal to produce a slave IF signal includes delaying the slave IF signal by a predetermined time.
There is further provided, in accordance with a preferred embodiment of the present invention a communications repeater system that includes a plurality of slave units, which receive a main radio frequency (RF) signal and a diversity RF signal transmitted by one or more mobile communication units in a vicinity of the slave units and which generate, responsive to the received RF signals, a respective main intermediate frequency (IF) signal and diversity IF signal, apparatus for converting the IF signals to respective recovered main and recovered diversity radio-frequency (RF) signals, including:
a main master unit, including:
a main IF port which receives the main IF signal;
a local oscillator which generates a local oscillator (LO) signal;
a reference signal generator, coupled to the local oscillator, which generates a reference signal responsive to the LO signal;
a main mixer coupled to the main IF port and the local oscillator which generates the recovered main RF signal responsive to the main IF signal and the LO signal; and
a main RF port coupled to the main mixer which transmits the recovered main RF signal; and
a diversity master unit, including:
a diversity intermediate frequency (IF) port which receives the diversity IF signal;
a receiver which generates a recovered LO signal from the reference signal received from the main master unit;
a diversity mixer coupled to the diversity IF port and the receiver which generates a recovered diversity RF signal responsive to the diversity IF signal and the recovered LO signal; and
a diversity RF port coupled to the diversity mixer which transmits the recovered diversity RF signal.
Preferably, the reference signal generator includes a frequency divider, which divides a frequency of the LO signal by an integer to produce a divided LO signal, and the receiver includes a multiplier which multiplies the divided LO signal by the integer to generate the recovered LO signal.
Preferably, the reference signal includes the LO signal, and the receiver includes a splitter which receives the LO signal and splits the LO signal to generate the recovered LO signal.
Preferably, each slave unit includes a respective IF signal forward path, wherein the forward path includes a delay element which delays the IF signal in the forward path by a predetermined time.
Further preferably, each slave unit includes a respective IF signal reverse path, wherein the reverse path includes a delay element which delays the IF signal in the reverse path by a predetermined time.
Preferably, the plurality of slave units includes:
one or more main slave units coupled to the main IF port which receive the main RF signal and generate the main IF signal responsive to the LO signal and the main RF signal; and
one or more diversity slave units coupled to the diversity RF port which receive the diversity IF signal and generate the diversity IF signal responsive to the LO signal and the diversity RF signal.
Preferably, the one or more main slave units are separated sufficiently in position from the one or more diversity slave units so that the main RF signal received by the one or more main slave units and the diversity RF signal received by the one or more diversity slave units are distinguishable from each other by the one or more main slave units and the one or more diversity slave units.
Preferably, the system includes an antenna coupled to the main RF port and the diversity RF port which radiates the main recovered RF signal and the diversity recovered RF signal.
Further preferably, the system includes a cable coupled to the main RF port and the diversity RF port which conveys the main recovered RF signal and the diversity recovered RF signal to a base transceiver station.
There is further provided, in accordance with a preferred embodiment of the present invention, a communications repeater system that includes a plurality of slave units, which receive a main radio frequency (RF) signal and a diversity RF signal transmitted by one or more mobile communication units in a vicinity of the slave units and which generate, responsive to the received RF signals, a respective main intermediate frequency (IF) signal, and diversity IF signal, a method for converting the main and diversity IF signals to respective recovered main and recovered diversity radio-frequency (RF) signals, including:
receiving the main IF signal generated responsive to the main RF signal in a main IF port;
generating a local oscillator (LO) signal;
mixing the main IF signal and the LO signal to generate a recovered main RF signal;
transmitting the recovered main RF signal from a main RF port;
receiving the diversity IF signal generated responsive to the diversity RF signal at a diversity IF port;
generating a reference responsive to the LO signal;
receiving the reference to generate a recovered LO signal;
mixing the diversity IF signal and the recovered LO signal to generate a recovered diversity RF signal; and
radiating the recovered diversity RF signal from a diversity RF port.
Preferably, generating the reference includes dividing a frequency of the LO signal by an integer to produce a divided LO signal, and receiving the reference includes multiplying the frequency of the divided LO signal by the integer.
There is further provided, in accordance with a preferred embodiment of the present invention, repeater apparatus, including:
first and second master units, which communicate with respective first and second wireless communication networks operating on respective first and second frequency bands;
a combiner, in wired communication with the first and second master units, so as to receive therefrom and convey thereto communication signals that are transmitted over the first and second networks; and
a plurality of slave units, coupled to the combiner, and located at a plurality of different, respective locations within a generally closed environment, so as to communicate with mobile wireless units operating on the first and second frequency bands.
Preferably, the first frequency band includes frequencies substantially in a range 800 MHz to 900 MHz and the second frequency band includes frequencies substantially in a range 1800 MHz to 1900 MHz.
Preferably, the plurality of slave units includes at least one slave unit operating on the first frequency band and at least one slave unit operating on the second frequency band.
Preferably, the repeater apparatus includes one or more antennas coupled at least to the first or the second master unit via which the respective master unit communicates with the respective wireless communication network.
Further preferably, the repeater apparatus includes a cable coupled to at least the first or the second master unit via which the respective master unit communicates with the respective wireless communication network.
Preferably, the first wireless communication network includes a cellular network, and the second wireless communication network includes a Personal Communication Services (PCS) network.
The present invention will be more fully understood from the following detailed description of the preferred embodiments thereof, taken together with the drawings, in which: